A method of detecting a torque acting on a detected object such as a rotation shaft in a non-contact manner includes using a torque detection device of the magnetostriction type. In the torque detection device, a pair of coils is wound around the detected object in a non-contact manner, and a pair of cores of the claw-pole type surrounding the inner and outer circumferences of them is concentrically provided without contacting the detected object. Each core is attached in such a manner that a pair of ring portions provided with pole teeth on the inner circumferential side at both ends of a tubular portion faces the detected object with the pole teeth arranged in a mutually meshing manner. The cores, each of which forms a magnetic path in conjunction with the detected object, are arranged symmetrically with respect to a line segment perpendicular to the axis of the detected object. A magnetic flux which is generated by energization of each coil causes a magnetic path to be formed by the core and the detected object.
A case where a torque has acted on a detected object is described. Depending on directions of a torque acting on the detected object, a compressive stress acts in a direction of +45° relative to the axis of the detected object and a tensile stress acts in a direction of −45° relative thereto, or a tensile stress acts in a direction of +45° relative thereto and a compressive stress acts in a direction of −45° relative thereto. Since a magnetic flux passes through the detected object in a direction inclined relative to the axis thereof, the magnetic flux passes therethrough in such a way as to travel along the direction of a tensile stress or compressive stress acting on the detected object. The relative magnetic permeability of the detected object varying causes a change in inductance of a pair of coils. Converting such a change in inductance of a pair of coils into a torque enables detecting a torque acting on the detected object. Moreover, as illustrated in FIG. 4, making clearances between teeth 101 and teeth 102, which mesh with each other along the vertical direction in the claw-pole type structure, different as clearances C1 and C2 enables setting the direction of a magnetic flux passing through the detected object to an intended direction (see PTL 1: Japanese Patent No. 5,683,001).